A communication method as a successor of the W-CDMA and the HSDPA, namely, the Long Term Evolution (LTE) has been considered by the W-CDMA standardization organization 3GPP, and the specification setting work is underway.
As a radio access method of the LTE, use of the OFDMA in the downlink and the SC-FDMA (Single-Carrier Frequency Division Multiple Access) in the uplink is under consideration.
The OFDMA is a method for dividing a frequency band into a plurality of narrow frequency bands (sub-carriers) and transmitting data loaded on the respective divided frequency bands. In this method, high-speed transmission can be achieved and frequency utilization efficiency can be improved by arranging sub-carriers densely on the frequency bands in such a manner that the sub-carriers partially overlap but do not interfere with each other.
The SC-FDMA is a transmission method which can reduce interference between terminals by dividing a frequency band and transmitting data by using frequency bands different among plural terminals. The SC-FDMA has a feature of less fluctuation in the transmission power, which can achieve low power consumption of terminals and wide coverage.
The LTE is a system in which a plurality of mobile stations communicate with each other by sharing one or more physical channels in both uplink and downlink.
A channel shared by a plurality of mobile stations is generally called a shared channel, which is, in the LTE system, a “Physical Uplink Shared Channel (PUSCH)” in the uplink and a “Physical Downlink Shared Channel (PDSCH)” in the downlink.
Also, such a shared channel is, as a transport channel, an “Uplink Shared Channel (UL-SCH)” in the uplink and a “Downlink Shared Channel (DL-SCH)” in the downlink.
In such a communication system using shared channels described above, it is necessary to select which mobile station UE the shared channel is to be allocated to, and to signal information indicating that the shared channel is allocated to the selected mobile station UE, for each sub-frame (1 ms, in the LTE).
In the LTE, a control channel used for the signaling is called the “Physical Downlink Control Channel (PDCCH)” or “Downlink L1/L2 Control Channel (DL L1/L2 Control Channel)”.
Meanwhile, the processing for each sub-frame of selecting which mobile station UE the shared channel is to be allocated to is generally called the “scheduling”. In this case, the processing may also be called the “dynamic scheduling”, because the mobile station UE to which the shared channel is allocated is dynamically selected for each sub-frame. Furthermore, “allocating the shared channel” described above may be alternatively expressed as “allocating a radio resource for the shared channel”.
Information of the physical downlink control channel includes, for example, the “downlink scheduling information”, “the uplink scheduling grant”, and the like.
The downlink scheduling information includes, for example, downlink resource block allocation information on the downlink shared channel, ID of UE, the number of streams, information on the precoding vector, data size, modulation scheme, information on the HARQ (hybrid automatic repeat request), and the like.
Meanwhile, the “uplink scheduling grant” includes, for example, uplink resource block allocation information on the uplink shared channel, ID of UE, data size, modulation scheme, uplink transmission power information, information on demodulation reference signal in the uplink MIMO, and the like.
Note that, the “downlink scheduling information” and the “uplink scheduling grant” described above may be collectively referred to as “downlink control information (DCI)”.
In the LTE system, the HARQ is applied to a communication using the shared channel described above. For example, on the downlink, the mobile station UE decodes the downlink shared channel and sends the radio base station eNB acknowledgement information (ACK/NACK) based on the decoding result (CRC check result) using the physical uplink control channel (PUCCH).
Then, the radio base station eNB performs retransmission control according to a content of the acknowledgement information, which is expressed either with a positive response (ACK) indicating that the transmitted signal is received properly or a negative response (NACK) indicating that the transmitted signal is not received properly.
FIG. 8 shows the downlink dynamic scheduling and the HARQ processing in the LTE mobile communication system described above.
In the sub-frame #3, the radio base station eNB transmits downlink scheduling information via the PDCCH and downlink data via the PDSCH to the mobile station UE.
Then, the mobile station UE receives downlink data via PDSCH on the basis of the downlink scheduling information received via PDCCH.
In the sub-frame #7, the mobile station UE transmits the acknowledgement information for the downlink data using PUCCH, and the radio base station eNB receives the acknowledgement information (ACK/NACK) mapped to the PUCCH.
The radio resource of the PDSCH described above is dynamically allocated as being notified by the PDCCH. Also, the radio resource of PUCCH described above is associated with the radio resource number of PDCCH described above. Dynamic allocation of such radio resource of PDCCH also results in dynamic allocation of such radio resource of PUCCH.
That is, in the normal downlink scheduling of the LTE mobile communication system, the radio base station eNB is configured to dynamically allocate a downlink radio resource and an uplink radio resource to the mobile station UE through the PDCCH, the downlink radio resource being for transmitting downlink data to the mobile station UE while the uplink radio resource being for transmitting acknowledgement information for the downlink data.
Furthermore, in the dynamic scheduling described above, a time difference is fixed between a sub-frame in which a PUCCH signal is transmitted and a sub-frame in which a PDCCH signal and a PDSCH signal are transmitted.
On the other hand, in the “persistent scheduling” that is under consideration to achieve the VoIP and the like, the radio base station eNB is configured to fixedly allocate the downlink radio resource (PDSCH) to a mobile station in first cycles starting from a sub-frame (first allocation starting time) in which the downlink scheduling information is transmitted to the mobile station UE through the PDCCH, and to fixedly allocate the uplink radio resource (PUCCH) to the mobile station UE through the upper layer (Radio Resource Control: RRC) signaling.
In the “persistent scheduling”, the downlink scheduling information is transmitted via PDCCH in a first transmission only, and the downlink scheduling information is not transmitted via PDCCH in the subsequent transmissions. For this reason, the method for associating a radio resource of the PUCCH with a radio resource number of the PDCCH cannot be applied unlike the “dynamic scheduling” described above.
Accordingly, the uplink radio resource (PUCCH) in the “persistent scheduling” is fixedly allocated to the mobile station UE by using the upper layer (RRC) signaling.
Here, the uplink radio resource represents, for example, a code resource in the code multiplexing or a frequency resource in the frequency multiplexing.
The frequency resource may be designated by a resource block number of a resource block (aggregate of sub-carriers) in which the PUCCH is transmitted.
Furthermore, when a plurality of pieces of acknowledgement information are multiplexed within one resource block, the uplink radio resource may be designated by a predetermined identification number. For example, an identification number may be used to designate an amount of cyclic shift in the multiplexing of CAZAC sequence cyclic shift or an orthogonal cover number in the block spreading.
Furthermore, generally in the HARQ, a time difference is fixed between a time point where the downlink radio resource (PDSCH) is allocated and a time point where the uplink radio resource (PUCCH) is allocated. Accordingly, a transmission timing of PUCCH is uniquely determined by designating a first allocation starting time through the PDCCH.
In the example shown in FIG. 9, the downlink radio resource (PDSCH) described above is fixedly allocated in cycles of 20 ms, and the uplink radio resource (PUCCH) is fixedly allocated for respective downlink radio resources (PDSCH).
Specifically, the downlink radio resource (PDSCH) is fixedly allocated in sub-frames #3, #23, . . . , while the acknowledgement information is transmitted in sub-frames #7, #27, . . . .
Note that, in the sub-frame #3 of the example shown in FIG. 9, a first allocation starting time is designated by PDCCH.
Here, in a prior art, a time difference between a time point where the downlink radio resource (PDSCH) described above is allocated and a time point where the uplink radio resource (PUCCH) is allocated, is provided to become a predetermined period of time (for example, four sub-frames).
Accordingly, when a timing of the downlink radio resource (PDSCH) allocated in the “persistent scheduling” is changed, a timing allocated to the uplink radio resource (PUCCH) is also changed similarly.
Note that, even in the case where a timing for allocating the uplink radio resource (PUCCH) is changed, an uplink radio resource (PUCCH) such as a code resource and a frequency resource is not changed. This is because, in the persistent scheduling, the uplink radio resource (PUCCH) such as a code resource in the code multiplexing and a frequency resource in the frequency multiplexing is fixedly allocated to the mobile station UE by using the upper layer signaling as described above.
At this time, if an uplink radio resource (PUCCH) to be allocated at a changed timing is already used by a different mobile station UE, the uplink radio resource (PUCCH) after the change collides with an uplink radio resource (PUCCH) of the different mobile station UE. Thus, there has been a problem that changing the timing of the downlink radio resource (PDSCH) allocated in the “persistent scheduling” cannot be processed.
In other words, there has been a problem that a timing of the downlink radio resource (PDSCH) allocated in the “persistent scheduling” cannot be changed freely, since a time difference is fixed between a time point where the downlink radio resource (PDSCH) is allocated and a time point where the uplink radio resource (PUCCH) is allocated, and the uplink radio resource (PUCCH) is set in advance.